1. Field of the Invention
The present invention relates to a forming method of a refractive index matching film used in an optical connector and, more particularly, to a forming method of a refractive index matching film that can reduce a connection loss caused by a temperature change and repetitive attachment and detachment of an optical fiber, and that can be formed easily. Furthermore, the present invention relates to an optical connector that uses the refractive index matching film.
2. Description of Related Art
When optical fibers are interconnected, ends of the both optical fibers are often made to mutually face or ferrules, into each of which an optical fiber is inserted, are made to mutually face. Connection members used in these types of connection include a mechanical splice and optical connector. The mechanical splice is effective for a long-term connection, and the optical connector is effective when the connection is frequently attached and detached. Regardless of which of these connection members is used, two optical fiber ends are mutually brought into physical contact, so a thrust force is applied in the axial direction of each optical fiber.
In these connection methods, the shape and/or state of the optical fiber end thus largely affects the connection characteristics. For example, if an angle of the optical fiber end relative to the axial direction of the optical fiber is not within an appropriate range or a state of a surface of the optical fiber end is rough, air remains at the clearance in the connection part, in which case Fresnel reflection at the connection end increases and thus the connection loss increases.
In known connection methods, as described in JP-A Hei 11 (1999)-72641 and JP-A Hei 11 (1999)-101919, to avoid this problem, a refractive index matching material in a liquid or grease state is applied to an end of an optical fiber to be connected, the refractive index of the refractive index matching material having the same or approximately the same refractive index as a core of the optical fiber. In this method, a refractive index matching material is applied to the end of an optical fiber to be connected or supplied into the connection part, and the optical fiber is made to face another optical fiber to be connected to the optical fiber. This method thereby keeps air out of their connection ends and avoids the Fresnel reflection that would otherwise be caused by air, reducing the connection loss. In other known methods as described in JP-A Hei 07 (1995)-294779, JP-A Hei 10 (1998)-221547 and JP-A-2005-275049, photo-curable resin is applied to the optical fiber end and cured instead of the refractive index matching material in a liquid or grease state being applied or supplied.
In the interconnection work of optical fibers, however, the optical fibers are often attached and detached repeatedly so that their optical axes are aligned. Accordingly, some problems arise in the conventional methods, as described below.
When optical fibers are repeatedly attached and detached in a method in which a silicone or paraffinic refractive index matching material in a liquid or grease state is applied to the connection end of one of the optical fibers, the amount of refractive index matching material between the optical fiber ends decreases and thus voids and bubbles are likely to be generated between the ends, greatly increasing a connection loss. Another problem is that the refractive index matching material may be lost due to volatilization, exudation, etc. over the long term. Furthermore, when holey fibers are connected, since the refractive index matching material is in a liquid or grease state, it is easy to enter hollows of the holey fibers, greatly changing the transfer loss of the holey fibers. In addition, since it reduces the amount of refractive index matching material in the connection part, voids and bubbles are likely to be generated between the ends, greatly increasing a connection loss.
In the method, described in JP-A Hei 07 (1995)-294779, in which photo-curable resin is applied to an optical fiber end and cured, an optical fiber end is immersed into in a liquid material and then pulled out upward. After that, a drop-shaped liquid material on the optical fiber end is cured. In this case, there is a problem that the liquid material is prone to adhere to the side of the optical fiber and becomes solidified. In addition, it is difficult to form a film with a desired thickness on the end surface.
As described in JP-A Hei 10 (1998)-221547, an optical fiber end is immersed into a vessel filled with photo-curable resin, light for curing the resin is emitted through the immersed fiber end, and then the distance from the bottom of the vessel to the optical fiber end is controlled so as to control the film thickness. In this method as well, however, there is a problem that the photo-curable resin adheres to the side of the optical fiber and becomes solidified. The document of JP-A Hei 10 (1998)-221547 also describes a method in which a cured material formed in a liquid is attached to the optical fiber end, rather than immersing the end into the vessel. However, it is difficult to control the shape of the cured material formed in the liquid.
As described in JP-A-2005-275049, an end of optical fiber is oriented upward and a liquid drop of photo-curable resin is applied to the end surface and cured. If an attempt is made to restrict the liquid drop within the end surface, the thickness of the resin is restricted. If the thickness of the resin is increased by repeatedly applying and curing liquid drops of the photo-curable resin, the liquid drops are prone to drip along the side.